1. Field
This invention relates to phased array antennas and, more particularly, to improvement in the distribution systems for such arrays.
2. Prior Art
Phased array antennas generally comprise a plurality of antenna elements in which the phase of the signal supplied to each element is controlled by one or more phase shifters. A series fed array is one in which the radiators are fed via paths which branch off a main transmission line so that this line is effectively in "series" with every path. Succeeding branches coupled to the main line receive a portion of the energy which has not been coupled to the preceeding branches. In a typical application, the main transmission line might be a waveguide and the branch lines might couple from the main guide via holes or slots in the main guide walls.
The series feed is termed traveling-wave type, mixed type, or standing-wave type depending on whether the VSWR in the main waveguide is low, intermediate or high respectively. The traveling wave type achieves the low standing wave ratio by use of a non-reflective termination and by use of branch junctions which introduce little reflection, either because they are lightly coupled to the main line or because the main line is appropriately adjusted to impedance match the junction. The spacing of the branches along the main line of a traveling wave series feed can be almost any value; an exception are values that are near n.lambda.g/2, where n=1, 2, 3 . . . etc., and .lambda.g=guide wavelength, that is, values which are resonant. For these resonant values, the reflections from each of the branch junctions are nearly in phase and add coherently to destroy the pure traveling-wave nature of the feed. Thus, the resonant spacing condition is avoided in a traveling wave type array. For other spacings, reflections from the branch junctions tend to add randomly, so that a nearly pure traveling-wave is maintained along the entire length of the main line.
The standing-wave type achieves the high standing wave ratio by use of a reflective termination and by restricting the coupling between main line and branches so that they only lightly load the main line. The branches are located periodically at voltage minima or maxima, depending on the type of coupling mechanism used; thus the branches are separated by "resonant" intervals of n.lambda.g/2 in the standing-wave type feed. Although this restriction limits the frequency bandwidth of standing-wave feeds, such feeds are desirable because the use of reflective terminations contributes to high feed efficiency.
Array feeds with resonant spacing between branches are also sometimes chosen because of their ability to "force-feed" radiator elements of appropriate types. A network is said to force feed radiators when the radiation field contribution from each radiator is specified by the network, independent of the way in which the radiators load the network. An important example of the use of force feeding within the prior art is the case where an array of full-wave dipoles is fed to obtain maximum directivity; in this case the optimum feed is by direct shunt along that line. To understand why this is optimum, first consider that the radiation field of a full-wavelength dipole can be shown to be independent of terminal impedance when it is excited by a specified terminal voltage. Thus, to guarantee that an array of such dipoles will produce a far field radiation pattern corresponding to that of a uniformly excited array, which provides maximum directivity, it is only required that the voltages along the transmission line at the branch points be equal.
Next consider how this equality is guaranteed. Campbell's formula in transmission line theory shows that loading a uniform line with pure series or pure shunt elements at multiple half-wavelength intervals produces no change in complex propagation constant. Thus radiating elements placed across the line exactly one guide wavelength apart are excited by voltages of exactly equal magnitude which are exactly in phase regardless of the admittance with which they load the line. This is true even if the loading is unequal because some radiators are near the end and others are near the middle of the array. In effect, for the resonant spacing between branch points, the radiators are simply in parallel with one another. Thus, a uniform excitation is forced and maximum directivity is achieved even though dipole terminal impedances are unequal.
The direction of the beam radiated from the series fed array is dependent on the progressive phase shift .phi., between branches along the main feed line as well as any additional progressive phase shift .psi. in the branch lines. More specifically, the wavefront radiated by the array is inclined to the plane of the array by the angle .theta., where sin .theta.=(.psi./l)(.theta.+.psi.)/2, (the value of .phi. being a function of frequency which is given by .phi.=2.pi.l/.lambda.g-n.pi.).lambda. is the free space wavelength, l is the interelement spacing and n is restricted to even values, unless adjacent branches alternatively contain 180 degree phase reversals, in which case n is restricted to odd values. Thus, generally the beam peak direction will be frequency dependent, unless this dependence is suppressed by arranging for .psi. to compensate for the variation in .phi. with frequency. The frequency dependence is sometimes used to scan the direction of the beam by simply varying the frequency of the signal. Alternatively, scanning is achieved by inserting phase shifters in either the branch lines or the main line to vary .psi. or .phi. respectively while operating at constant frequency. Scanning by varying .phi. has an advantage over varying .psi. in that all the phase shifters are driven to the same setting, and this setting is a smaller value of phase shift than the average setting of a branch-located phase shifter because the main-line phase shifts are in series and therefore are additive.
In the prior art of standing wave arrays, .phi. could not be varied to scan the array since this would violate the resonant spacing condition needed for coupling off the main line at voltage minima or maxima. Thus, the advantages of scanning with .phi. have been restricted to cases where traveling wave feeds are used. Even here, usage has been restricted by the need to avoid the resonant spacing with the prior art traveling wave feed, as explained earlier. Resonant spacing corresponds to a broadside scan angle, where .phi.=0, assuming .psi.=0, which is the useful case corresponding to identical branch lines. For this reason, traveling wave arrays of the prior art cannot scan close to the broadside direction. Also if it were desired to force feed the prior art array, the need for resonant spacing rules out the possibility of scanning the array by changing .phi.. Other considerations rule out scanning by changing .psi. if force feed is to be maintained.
The description of prior art above has concentrated on traveling-wave and standing wave type examples. Several examples of what has previously been termed a mixed-type feed are also in the prior art. One of particular interest is the case in which the series-feed main-line ends with a nonreflective termination and is constructed with branches at resonant spacings. Such an array could have greater bandwidth that the standing wave type, and yet reap the advantages of force feed. However, its use within the prior art is restricted to cases where the array is not scanned in order to maintain the resonant spacing.